In order to study charge trapping and transport in polyimide, we have submitted samples of Kapton HN to corona charging and measured its surface potential decay with an electrostatic voltmeter. We propose a two terms mobility to explain the experimental data: a non dispersive contribution based on Toomer and Lewis model added to a dispersive term, which is associated with the disordered structure of the material. The non-dispersive model alone did not fit well to the data for short times, but our assumption makes the theoretical expresion fit succesfully to the experimental data. Some important parameters of the material are determined and discussed.